If you are looking for a logo for the front page or clipart for your website, you want to check out the openclipart project. This project aims to create an archive of user contributed clip art that can be freely used.
There are no limitations on usage. All graphics that have been submitted by the artists are placed into the Public Domain.
The project collects the public-domain clipart in a single large download and browsable html pages. The clipart is stored in Scalable Vector Graphics (SVG) format, often with thumbnails in Portable Network Graphics (PNG) format.
You can download the +12,000 pictures here. The project started in early 2004 by the developers of Inkscape - you can find more information about Inkscape here.
You may also want to check out the Wikimedia Commons media file repository that makes available public domain and freely-licensed educational media content (images, sound and video clips).
Launched on 7 September 2004, Wikimedia Commons hit the 1,000,000 uploaded media file milestone on 30 November 2006 and currently contains over 5,582,526 files and 93,622 media collections.